1. Field of the Invention
The present invention relates to a thyristor turn-off system for shortening the turn-off period and simplifying the turn-off means.
2. Description of the Prior Art
As is known, a turn-off of the conventional thyristor has been performed by reverse-biasing between an anode and a cathode for a specific period. According to the conventional turn-off system, a turn-off period must usually be greater than 10 .mu. sec.. It has been difficult to shorten the turn-off period for a current thyristor of higher than 50 A or a high voltage thyristor, for example, higher than 800 V. Accordingly, complicated turn-off means has been required and the loss for the turn-off has been large. The disadvantages have been great in the case of high frequency switching.
As is known in the conventional gate turn-off thyristor, the thyristor is turned off by reverse-biasing the gate. It has been hard to use the gate turn-off thyristor for large current such as greater than 10 A or for high voltage such as higher than 600 V. On the other hand, it has been difficult to use a transistor for current of greater than about 30 A and high voltage of higher than 300 V. Even though the static break over voltage V.sub.ceo or V.sub.cbo can be higher than 300 V, it has been difficult to use it for high speed high frequency switching because of the break down by switching power.
An allowable surge current of a transistor is smaller than that of a thyristor. Accordingly, it is not feasible to use a transistor for a high power switching purpose. Accordingly, it has not been possible to overcome the disadvantages of a thyristor by a transistor.